A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive traces, pads and other features etched from electrically conductive sheets, such as copper sheets, laminated onto a non-conductive substrate. Multi-layered printed circuit boards are formed by stacking and laminating multiple such etched conductive sheet/non-conductive substrate. Conductors on different layers are interconnected with plated-through holes called through-hole vias.
A printed circuit board includes a plurality of stacked layers, the layers made of alternating non-conductive layers and conductive layers. The non-conductive layers can be made of prepreg or base material that is part of a core structure, or simply core. Prepreg is a fibrous reinforcement material impregnated or coated with a resin binder, and consolidated and cured to an intermediate stage semi-solid product. Prepreg is used as an adhesive layer to bond discrete layers of multilayer PCB construction, where a multilayer PCB consists of alternative layers of conductors and base materials bonded together, including at least one internal conductive layer. A base material is an organic or inorganic material used to support a pattern of conductor material. A core is a metal clad base material where the base material has integral metal conductor material on one or both sides. A laminated stack is formed by stacking multiple core structures with intervening prepreg and then laminating the stack. A through-hole via is then formed by drilling a hole through the laminated stack and plating the wall of the hole with electrically conductive material, such as copper. The resulting plating interconnects the conductive layers in the laminated stack.
In order to lower overall board thickness after assembly, inclusion of a recessed cavity in the PCB is getting more attention in consumer electronic and telecommunication products. Common fabrication process is to pre-cut low flow prepreg at a cavity area and then control resin squeeze out during the lamination process. This process has disadvantages such as high cost of low flow prepreg, limited supply of low flow prepreg and difficulty in controlling resin squeeze out into the cavity. Additionally, lamination accessories such as release film and conformal film are needed which also add cost. Release film provides a separation between a surface copper layer (conducting layer) in the lamination stack and the conformal film. Conformal film is a thermoplastic layer which softens under lamination temperature and conforms to the area with prepreg pre-cut. Use of low flow prepreg requires higher lamination pressure. Lamination under high pressure and the impact of conformal film can result in increased panel distortion and it is difficult to achieve flat surface for fine line etching or even dielectric thickness across the panel to control impedance. A panel here refers to the finished product of the stack of laminate and prepreg after lamination. In order to solve these issues, a new manufacturing process for forming a cavity within a PCB is needed.
A recessed cavity can be a single layer, few layers or many layers deep into the laminated stack-up of the PCB. One or more of the conductive layers through which the recessed cavity is formed may be interconnected by plating the side walls and bottom surface of the recessed cavity. In some applications, it is desired that the recessed cavity bottom surface, and possibly one or more conductive layers closest to the recessed cavity bottom surface, are not plated and therefore not electrically connected to other conductive layers using the recessed cavity side wall plating. In this case the recessed cavity is a disconnect cavity. One such application is directed to a radio frequency PCB board, where it is common to have a recessed cavity deep within the laminated stack-up of the PCB, with the deepest portion of the recessed cavity forming a transmission cavity. For proper transmission wave propagation through the transmission cavity, the transmission cavity must be clean with nothing that may interfere or distort a transmission wave. In some designs, the transmission cavity wall has to be disconnected from the PCB landing layer, or outer surface to which components are mounted. A possible method for forming such a disconnect cavity is to use pre-cut low flow prepreg at a disconnect cavity area and selective copper etching to disconnect transmission cavity wall from the landing layer. However, it is difficult to control low flow prepreg resin flow into the disconnect cavity. Additionally, selective copper etching to disconnect transmission cavity wall from the landing layer is not effective.